Workers at a company operating windfarms formerly owned by the Quinn Group have been warned to stay away or "face the bullet".

A chilling handwritten message, accompanied by three bullets, was left near the company's premises during the week. The message ends with the words: "Final warning."

A spokesman for the company concerned, Vestas, confirmed it had received a warning letter and said it is engaging with the relevant authorities.

A cross-border police investigation is underway following a fresh outbreak of hostilities over the former Quinn Group.

Police in Northern Ireland have warned Liam McCaffrey, the chief executive of Quinn Industrial Holdings (QIH) - which bought some of the Quinn Group's former assets - to review his personal security in recent weeks, according to informed sources. It is understood that a director of the company, John McCartin, a Fine Gael councillor, has been advised on security by gardai.

Mr McCaffrey was a key player in bringing Sean Quinn back into the fold after a group put together by Quinn supporters bought back some of the businesses he was ousted from over his massive debts. Mr Quinn returned to the group's old headquarters amid jubilant scenes in late 2014 and was appointed as a consultant to the business.

It is understood that police believe the threats directed against senior management in the company are of a general nature rather than specific.

Investors who financed the management team's takeover of the former Quinn Group companies, have also been briefed on the police investigations into the alleged threats.

The developments mark a sinister return to the threats and intimidation that dogged the Quinn Group companies after the bank placed them in receivership in 2011. It follows a prolonged period of calm that started when his former management team, backed by US investors, took over former Quinn Group packaging and building businesses in 2014.

Sources say that in recent months, differences have emerged between Sean Quinn and the management team over the future of Quinn Industrial Holdings, and Mr Quinn's role in the business.

A local group called Concerned Irish Citizens, which led a Facebook campaign to return the old Quinn Group companies to their founder, began posting messages critical of the management team at QIH earlier this year.

Mantlin, the owner of a former Quinn windfarm in the area, said it was "very concerned" about the message the incidents were sending out: "As an EU company which has invested in the Northern Ireland energy market, we are very concerned about the message these aggravated attacks are sending out about investment in Fermanagh."

In a statement, a spokesperson for the Quinn family said: "Sean has repeatedly condemned acts of sabotage and intimidation made against Quinn companies and management, his statements were widely reported in the national media at that time."

"Unfortunately, in recent times, certain security firms have again been engaged by some of the parties, their involvement has previously led to an escalation of tension."